Spread spectrum microwave overlay with notch filter

ABSTRACT

A spread spectrum CDMA communications system for communicating data and/or digitized voice between a plurality of users to a plurality of PCN units. The spread spectrum communications system is located within a same geographical region as occupied by an existing microwave system. The spread spectrum can operate without causing undue interference to the microwave system and the microwave system can operate without causing significant interference, less than 1% total users affected, to the PCN system. The spread spectrum CDMA communications system includes a plurality of PCN-base stations and a plurality of PCN units. A PCN-base station has a device for converting the format of the data into a form suitable for communicating over radio waves, a spread spectrum modulator for spread spectrum processing the data, a notch filter, and a transmitter for transmitting the spread spectrum processed converted data from the PCN-base station to a PCN unit. The PCN-base station also has an antenna, and spread spectrum detectors for recovering data communicated from the PCN units. A PCN unit has an antenna, and a detector coupled to the antenna for recovering data communicated from the PCN-base station. The detector includes a spread spectrum demodulator. Also, the PCN unit has a device for converting the format of the data into a form suitable for communicating over radio waves, spread spectrum modulator, an adjustable notch filter and a transmitter.

BACKGROUND OF THE INVENTION

This invention relates to spread spectrum communications and moreparticularly to a personal communications network which communicatesover the same spectrum as used by a plurality of existing narrowbandmicrowave users.

DESCRIPTION OF THE PRIOR ART

The current fixed service, microwave system uses the frequency band1.85-1.99 GHz. Microwave users in this frequency band typically have abandwidth of 10 MHz or less.

A problem in the prior art is the limited capacity of the channel, dueto the number of channels available in the fixed service, microwavesystem.

OBJECTS OF THE INVENTION

An object of the invention is to provide a personal communicationsnetwork (PCN) for increasing capacity for communications in a fixedservice microwave system environment.

Another object of the invention is to provide a PCN system which can beused at the same frequencies as used by the fixed service, microwavesystems.

An additional object of the invention is to provide a PCN system whichcan be used concurrently with a fixed service microwave system withoutinterfering with the fixed service microwave system.

A further object of the invention is a PCN system which allowscommunications between a base station (cell) and PCN users with spreadspectrum.

A still further object of the invention is a PCN system which canoverlay geographically and overlay in spectrum, on an already existingfixed service, microwave system, without modifications to the fixedservice, microwave system.

SUMMARY OF THE INVENTION

According to the present invention, as embodied and broadly describedherein, a spread spectrum code division multiple access (CDMA)communications system for communicating data between a plurality of PCNusers is provided comprising a plurality of PCN-base stations and aplurality of PCN user units. The PCN users communicate through thePCN-base station. Data may be, but are not limited to, computer data,facsimile data or digitized voice. In addition, such users might, insome applications, communicate directly to one another as in apoint-to-point or local area network (LAN) configuration.

The spread spectrum CDMA communications system is located within ageographical region, cell, overlaying a fixed service, microwave system.The fixed service, microwave system transmitter and receiver, and thePCN-base station are not collocated. Each of the fixed service,microwave systems communicate over a fixed-service, microwave channelwhich has a fixed-service bandwidth. Typically, the fixed-servicebandwidth is 10 MHz or less.

In the 1.85-1.99 GHz region, the spectrum is divided into a plurality offixed-service, microwave channels. A first fixed-service microwavechannel of a first microwave system is separated from a secondfixed-service microwave channel of a second microwave system by afrequency guard band and/or geographical separation and/or spatialseparation caused by narrow beam antennas.

A plurality of PCN-base stations overlay the same geographical region asoccupied by the fixed service, microwave system. A PCN-base stationcommunicates data between the plurality of PCN users. A PCN user uses aPCN unit, which may be embodied as a PCN handset.

Each PCN-base station has base-converting means, base-product-processingmeans, base-transmitting means, a base antenna, and base-detectionmeans. Each base station optionally may have base-filter means. Thebase-converting means converts the format of the data to be transmittedto a PCN user into a form suitable for communicating over radio waves.The base-product-processing means processes the data with spreadspectrum modulation. The base-filter means inserts one or more notchesin the power spectrum transmitted from the base station. The notches arelocated at the same frequency as a fixed-service, microwave channel, andtypically have approximately the same bandwidth as the fixed-servicebandwidth. The base-transmitting means transmits, with a spread-spectrumbandwidth, from the PCN-base station to a PCN unit, thespread-spectrum-processed-converted data. The spread-spectrum bandwidthcan overlay in frequency, one or more, fixed-service microwave channels,and is much greater than the bandwidth of a single fixed-servicemicrowave channel.

The base-detection means is coupled to the base antenna. Thebase-detection means recovers data communicated from the PCN unit to thePCN-base station.

All the users lie in cells. However, some users are in a first cell andother users are in a second cell, etc. PCN units are located in cells.Each of the PCN units has a PCN antenna and PCN detection means. ThePCN-detection means recovers data communicated from the PCN-basestation. For communicating to the PCN-base station, the PCN unit hasPCN-converting means, PCN-product-processing means, PCN-filter means andPCN-transmitting means. The PCN-converting means converts the format ofdata from a PCN user into a form suitable for communicating over radiowaves. The PCN-product-processing means processes the data with spreadspectrum modulation. The PCN-filter means inserts one or more notches inthe power spectrum transmitted from the PCN unit at the same frequencyas one or more fixed-service, microwave channels. The bandwidth of anotch filter typically is approximately the same as the fixed-servicebandwidths. The PCN-transmitting means transmits with a spread-spectrumbandwidth, the spread spectrum processed converted data from the PCNunit to a PCN-base station. The spread-spectrum bandwidth from thePCN-transmitting means is the same as the base-transmitting means, andmay partially or totally overlay the frequency spectrum of a microwavesignal, or partially or totally overlay the spectrum of differentmicrowave signals.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention also may be realized andattained by means of the instrumentalities and combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred embodiments of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 is a block diagram of a PCN-base station receiver;

FIG. 2A is a block diagram of a first embodiment of a PCN-base stationtransmitter;

FIG. 2B is a block diagram of a second embodiment of a PCN-base stationtransmitter;

FIG. 2C is a detailed block diagram of a transmitter;

FIG. 3 is a block diagram of a PCN-unit receiver;

FIG. 4A is a block diagram of a first embodiment of PCN-unittransmitter;

FIG. 4B is a block diagram of a second embodiment of a PCN unittransmitter;

FIG. 4C is a detailed block diagram of a PCN transmitter;

FIG. 5 shows the spectrum of a spread spectrum signal with an AM signalof equal power at its carrier frequency;

FIG. 6 shows a spread spectrum data signal when the spread spectrumsignal power is equal to an AM signal power;

FIG. 7 shows an audio signal when the spread spectrum signal power isequal to the AM signal power;

FIG. 8 shows a possible pseudo-random sequence generator;

FIG. 9 shows possible position settings of switches of FIG. 8 to form PNsequences;

FIG. 10 illustrates a PCN system geographic architecture according tothe present invention;

FIG. 11 shows fixed service microwave and PCN user geometry and urbanpropagation models;

FIG. 12 illustrates a typical fixed service microwave user antennapattern versus elevation angle;

FIGS. 13A and 13B shows link parameters for a typical 2 GHz fixedservice link and a typical PCN system;

FIG. 14 illustrates calculated in-band received power at a fixed servicemicrowave receiver in the presence of PCN users;

FIG. 15 depicts the region where a PCN handset has an error rate,P_(e) >10 ⁻², due to fixed service microwave transmission and PCN versushandset cell range;

FIG. 16 shows a PCN field test experiment;

FIGS. 17A-17K show measured attenuation versus distance; and

FIG. 18 shows the spectrum of a spread spectrum signal with multipath.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals indicate likeelements throughout the several views.

This patent is related to U.S. Pat. application having Ser. No.07/622,235, filing date Dec. 5, 1990, entitled SPREAD SPECTRUM CDMACOMMUNICATIONS SYSTEM by Donald L. Schilling, and to U.S. Pat.application having Ser. No. 07/626,109, filing date Dec. 14, 1990,entitled SYNCHRONOUS SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD,which are both incorporated herein by reference.

The spread spectrum CDMA communications system of the present inventionis located within a same geographical region as occupied by at least onefixed service, microwave system or other microwave system. Each fixedservice microwave system communicates over a fixed-service microwavechannel, which has a fixed-service bandwidth. In presently deployedfixed service microwave systems, the fixed-service bandwidth is 10 MHzor less.

In the 1.85-1.99 GHz region, the spectrum is used by a plurality ofnarrowband users, with each microwave user using one of a plurality offixed-service-microwave channels. A first fixed-service microwave systemusing a first fixed-service microwave channel is separated in frequencyby a guard band from a second fixed-service microwave system using asecond fixed-service microwave channel. The first fixed-servicemicrowave system may be separated geographically or spatially from thesecond fixed-service microwave system.

The spread spectrum CDMA communications system, which uses directsequence (DS) spread spectrum modulation includes a plurality ofPCN-base stations and a plurality of PCN units located within the samegeographical region as occupied by the plurality of microwave users ofthe fixed service microwave system. The spread spectrum CDMAcommunications system can be used for communicating data between aplurality of PCN users. The data may be, but are not limited to,computer data, facsimile data or digitized voice.

A PCN-base station, which typically is not collocated geographicallywith a fixed service microwave station, communicates data between aplurality of PCN users. A first PCN user uses a first PCN unit, and asecond PCN user uses a second PCN unit, etc.

Each PCN-base station includes base-converting means,base-product-processing means, base-transmitting means, base-detectionmeans and a base antenna. Each base station optionally may havebase-filter means. The base-detection means may includebase-spread-spectrum-processing means and base-synchronizing means. Thebase-detection means broadly is a repeater which converts spreadspectrum coded data communicated from one PCN unit into a form suitablefor another PCN user or telecommunication user.

Each of the PCN-base stations may be geographically spaced such that thepower radiated by the base-transmitting means from within its cell up toa contiguous cell of a neighboring PCN-base station primarily variesinversely with distance by an exponent of approximately two, and thepower radiated by the base-transmitting means outside its cell primarilyvaries inversely with distance by an exponent which is greater than two,typically four or more.

The geographic spacing of cells typically is small, on the order of 1200to 2000 feet. The small spacing allows the use of low transmitter power,so as not to cause interference with the fixed-service microwavesystems. Also, an in-band fixed service microwave user is oftenspatially and geographically distant from the PCN system, and when thisoccurs it results in negligible interference with the fixed-servicemicrowave user. As set forth below, base-filter means inserts notches inthe power spectrum transmitted from the base-transmitting means, whichessentially eliminates all interference from the base station to afixed-service microwave system.

The base-spread-spectrum-processing means, as illustrated in FIG. 1, maybe embodied as a pseudorandom generator, a plurality of product devices141 and a plurality of bandpass filters 143. The pseudorandom generatorstores chip codes, g₁ (t), g₂ (t), . . ., g_(N) (t), for demodulatingdata from spread spectrum signals received from the plurality of PCNunits at the PCN-base station. The base-detection means also includesmeans for synchronizing the base-spread-spectrum-processing means toreceived spread spectrum signals.

The base-spread-spectrum-processing means at the PCN-base stationprocesses selected data received from a selected PCN unit, which weretransmitted with a spread spectrum signal using a selected-chip code,g_(i) (t). The detector 145 demodulates the selected data from thedespread spread-spectrum signal.

A plurality of product devices 141, bandpass filters 143 and detectors145 may be coupled through a power splitter 147 to an antenna 149, forreceiving simultaneously multiple spread-spectrum channels. Each productdevice 141 would use a selected chip code for demodulating a selectedspread spectrum signal, respectively.

For a spread spectrum system to operate properly, the spread spectrumreceiver must acquire the correct phase position of the received spreadspectrum signal, and the receiver must continually track that phaseposition so that loss-of-lock will not occur. The two processes ofacquisition and tracking form the synchronization subsystem of a spreadspectrum receiver. The former operation is typically accomplished by asearch of as many phase positions as necessary until one is found whichresults in a large correlation between the phase of the incoming signaland the phase of the locally generated spreading sequence at thereceiver. This former process occurs using correlator means or matchedfilter means. The latter tracking operation is often performed with a"delay-locked loop". The importance of the combined synchronizationprocess cannot be overstated for if synchronization is not both achievedand maintained, the desired signal cannot be despread.

The base-converting means, as illustrated in FIG. 2A, may be embodied asa plurality of base modulators 151. A base modulator 151 converts theformat of data to be transmitted to a PCN user into a form suitable forcommunicating over radio waves. For example, an analog voice signal maybe converted to a base-data signal, using a technique called sourceencoding. Typical source coders are linear predictive coders, vocoders,delta modulators and pulse code modulation coders.

The base-product-processing means may be embodied as a plurality ofbase-spread-spectrum modulators 153. A base-spread-spectrum modulator153 is coupled to a base modulator 151. The base-spread-spectrummodulator 153 modulates the converted-data signal using spread spectrum.The converted data is multiplied using a product device or modulo-2added, using an EXCLUSIVE-OR gate 153 with a selected spread-spectrumchip code, g_(N+i) (t), as shown in FIG. 2B. The spread-spectrumbandwidth of the converted data is much greater than, at leastapproximately two times, the narrowband bandwidth of a fixed servicemicrowave user. The spread-spectrum bandwidth typically overlays inspectrum one or more fixed-service channels. In a preferred embodiment,the spread-spectrum bandwidth is 48 MHz.

The base-transmitting means may be embodied as a plurality of basetransmitters 155. A base transmitter 155 is coupled to abase-spread-spectrum modulator 153. The base transmitter 155 transmitsacross the fixed service microwave bandwidth, thespread-spectrum-processed-converted data from the PCN-base station to aPCN unit. The base transmitter 155 includes modulating the spreadspectrum processed converted data at a carrier frequency, f_(o).

The base-transmitter 155 has a transmitter oscillator which supplies acarrier signal at the carrier frequency. The transmitter oscillator iscoupled to a transmitter product device. The transmitter multiplies,using the transmitter-product device, thespread-spectrum-processed-converted data by the carrier signal. A moredetailed description of transmitter 155 is provided in FIG. 2C.

The base-transmitting means may, in a preferred embodiment, transmitdata using a spread spectrum signal having a power level limited to apredetermined level. The base-transmitting means may transmit data byadding the plurality of spread spectrum data signals.

A plurality of modulators 151, product devices 153 and transmitters 155may be coupled through a power combiner 157 to an antenna 159 forsimultaneously transmitting a multiplicity of spread-spectrum channels.FIG. 2A is an illustrative embodiment for generating simultaneous spreadspectrum signals, and there are many variants for interconnectingproduct devices, modulators and transmitters, for accomplishing the samefunction.

As an alternative example, FIG. 2B illustrates a PCN-base stationtransmitter which may be used for producing the same result as thetransmitter of FIG. 2A. In FIG. 2B data are modulo-2 added, usingEXCLUSIVE-OR gates 253 with a selected spread-spectrum chip code,g_(N+i) (t). The resulting spread-spectrum-processed data from aplurality of EXCLUSIVE-OR gates 253 are combined using combiner 257. Thebase transmitter 255 modulates the combined spread-spectrum-processeddata at the carrier frequency, f_(o). The transmitter 255 is coupled tothe antenna 159 and simultaneously transmits the plurality ofspread-spectrum-processed data as a multiplicity of spread-spectrumchannels.

FIG. 2C illustratively shows the base-filter means embodied as a notchfilter 525, as part of a PCN-base transmitter. The embodiment shown inFIG. 2C may be employed in the base transmitters 155, 255 of FIGS. 2Aand 2B. The notch filter 525 inserts one or more notches in the powerspectrum transmitted from the base-transmitting means. The notches arelocated at the same frequency as a fixed-service, microwave channel, andtypically have the same bandwidth, a fixed-service bandwidth, as thefixed-service microwave channel. Preferably, the notches provide 15 dBor more attenuation.

The notch filter 525 can be implemented at an intermediate frequency ofthe transmitter, or with technology permitting, the notch filter couldoperate at the carrier frequency, f_(o). The notch filter 525 is shownas an example, in FIG. 2C, coupled between a first transmitter mixer 522and a second transmitter mixer 524. The first transmitter mixer 522 iscoupled to a first local oscillator 521, and the second transmittermixer 524 is coupled to a second local oscillator 523. A transmittertypically has a power amplifier 528 coupled to an output of the secondlocal oscillator 523.

In FIG. 2C, the first local oscillator 521 supplies a signal to thefirst transmitter mixer 522 for modulating the combinedspread-spectrum-processed data from combiner 257. The second localoscillator 523 supplies a signal to the transmitter mixer 524 formodulating the notched combined spread-spectrum-processed data,outputted from the notch filter 525, to the carrier frequency.

Assume that the bandwidth of the combined spread-spectrum-processed datais much greater than that of fixed-service microwave user's bandwidth.The notch filter 525 can insert notches in the spectrum of the combinedspread-spectrum-processed data such that when the combinedspread-spectrum-processed data are modulated and transmitted at thecarrier frequency, f_(o), the notches coincide with the fixed-service,microwave channels.

Typically, a PCN-base station and the fixed-service microwave stationhave fixed geographic locations, and the fixed-service microwave channelis at a preassigned frequency and bandwidth. Thus, a notch filter for aPCN-base station can be a fixed design. The notch filter at a PCN-basestation alternatively may be an adjustable notch filter. The adjustablenotch filter can be responsive to a dynamic environment, where microwavesignals or channels appear unexpectedly.

The notch in the spectrum of the transmitted spread-spectrum signalsfrom the PCN-base station is less than the bandwidth of the spectrum.For example, the transmitted spread-spectrum signals from a PCN-basestation might have a bandwidth of 48 MHz. The fixed-service, microwavechannel might have a fixed-service bandwidth of less than 10 MHz. Thus,in this example, a notch filter would reduce the energy in thetransmitted spread-spectrum signal from the PCN-base station byapproximately only 20% or less.

The present invention also includes PCN units which are located withinthe cell. Each of the PCN units has a PCN antenna, PCN-detection means,PCN-converting means, PCN-product-processing means, PCN-filter means andPCN-transmitting means. The PCN-detection means is coupled to thePCN-antenna. The PCN-detection means includesPCN-spread-spectrum-processing means.

The PCN-detection means recovers data communicated to the PCN unit fromthe PCN-base station. The detection means also includes means forconverting the format of the data for output to a user. The format maybe, for example, computer data, an analog speech signal or otherinformation. The PCN-detection means, by way of example, may includetracking and acquisition circuits for the spread spectrum signal, aproduct device for despreading the spread spectrum signal and anenvelope detector. FIG. 3 illustratively shows an antenna 169 coupled toPCN detection means, which is embodied as a PCN spread-spectrumdemodulator 161, PCN-bandpass filter 163, and PCN-data detector 165.

The PCN-spread-spectrum demodulator 161 despreads, using a chip-codesignal having the same or selected chip code, g_(N+i) (t), as thereceived spread-spectrum signal, the spread-spectrum signal receivedfrom the PCN-base station. The bandpass filter 163 filters the despreadsignal and the PCN-data detector 165 converts the format of the despreadspread-spectrum signal for output to a PCN user.

The PCN-spread-spectrum-processing means includes means for storing alocal chip code, g_(N+i) (t), for comparing to signals received forrecovering data sent from the PCN-base station to the PCN unit.

The PCN-spread-spectrum-processing means also may include means forsynchronizing the PCN-spread-spectrum-processing means to receivedsignals. Similarly, the PCN-spread-spectrum-processing means at thePCN-base station includes means for processing data for particular PCNunits with a selected chip code.

The PCN-converting means, as illustrated in FIG. 4A, may be embodied asa PCN modulator 171. The PCN modulator 171 converts the format of thedata into a form suitable for communicating over radio waves. Similar tothe PCN-base station, an analog voice signal may be converted to aconverted-data signal, using a technique called source encoding. As withthe base modulator 151, typical source encoders are linear predictivecoders, vocoders, adaptive delta modulators and pulse code modulators.

The PCN-product-processing means may be embodied as aPCN-spread-spectrum modulator 173. The PCN-spread-spectrum modulator 173is coupled to the PCN modulator 171. The PCN-spread-spectrum modulator173 modulates the converted-data signal with a selected chip code, g_(i)(t). The converted-data signal is multiplied using a product device withthe selected chip code, g_(i) (t). The spread-spectrum bandwidth of theconverted data is much greater than, approximately five times greater inthe preferred embodiment, the narrowband bandwidth of a fixed servicemicrowave user. In a preferred embodiment, the spread-spectrum bandwidthis 48 MHz. The spread-spectrum bandwidth from the PCN modulator 171 isthe same as that from the modulator 151 at the PCN-base station, and mayoverlay the same microwave frequency or overlay separate microwavefrequencies.

As an equivalent transmitter, FIG. 4B illustrates a transmitter for aPCN unit having PCN-spread-spectrum-processing means as a PCN modulo-2adder, embodied as an EXCLUSIVE-OR gate 273. The EXCLUSIVE-OR gate 273modulo-2 adds the converted data signal with the selected chip code,g_(i) (t).

The PCN-transmitting means in FIGS. 4A and 4B may be embodied as a PCNtransmitter 175. The PCN transmitter 175 is coupled between thePCN-spread-spectrum modulator 173 and antenna 179. The PCN transmitter175 transmits across the fixed-service microwave bandwidth, thespread-spectrum-processed-converted data from the PCN unit to thePCN-base station. The PCN transmitter 175 modulates thespread-spectrum-processed-converted data at a carrier frequency, f_(o).The carrier frequency of the PCN transmitter and the cell transmittermay be at the same or at different frequencies. Typically the PCNtransmitter and the cell transmitter use the same frequency if halfduplex is used, and two frequencies if full duplex is used.

The PCN-filter means inserts one or more notches in the power spectrumtransmitted from the PCN-transmitting means. The notches are located atthe same frequency as a fixed-service, microwave channel, and typicallyhave the same bandwidth as a fixed-service microwave channel.Preferably, the notches provide 15 dB or more attenuation.

FIG. 4C illustrates the PCN-filter means embodied preferably as anadjustable-notch filter 725 as part of a PCN transmitter. The embodimentshown in FIG. 4C may be employed in the PCN transmitter 175 of eitherFIG. 4A or FIG. 4B. The adjustable notch filter 725 is shown implementedat an intermediate frequency of the PCN transmitter 175, although withtechnology permitting, the adjustable notch filter 725 can be at thecarrier frequency, f_(o), of the PCN transmitter 175.

The adjustable-notch filter 725 is coupled between a firstPCN-transmitter mixer 722 and a second PCN-transmitter mixer 724. Thefirst PCN-transmitter mixer 722 is coupled to a first PCN-localoscillator 721, and the second PCN-transmitter mixer 724 is coupled to asecond PCN-local oscillator 723. A transmitter typically has a poweramplifier 728 coupled to an output of the second local oscillator 723.

In FIG. 4C the first PCN-local oscillator 721 provides a firstoscillator frequency signal to the first PCN-transmitter mixer 722. Thefirst PCN-transmitter mixer 722 modulates the spread-spectrum-processeddata to the PCN-transmitter intermediate frequency, f_(IF). The secondPCN-local oscillator 723 provides a second oscillator signal to thesecond PCN-transmitter mixer 724. The second PCN-transmitter mixer 724modulates the notched spread-spectrum-processed data to a carrierfrequency, f_(o).

Assume that the bandwidth of the spread-spectrum-processed data is muchgreater that of the fixed-service bandwidth. The adjustable-notch filter725 can insert notches in the spectrum of the spread-spectrum-processeddata such that when the spread-spectrum-processed data are modulated andtransmitted at the carrier frequency, f_(o), the notches coincide withthe fixed-service microwave channels.

A PCN unit is assumed to roam within a geographic region of one or morecells. Thus, the PCN unit, at different locations, may tend to interferewith fixed-service, microwave channels at different frequencies Theadjustable-notch filter 725 has its center frequency and bandwidth setso as to notch the power spectrum from the PCN transmitter at whateverdesired frequency and bandwidth of the fixed-service, microwave channel.

The adjustable-notch filter 725 can be controlled several ways. First,each PCN-base station can be programmed with the frequency and bandwidthof each fixed-service microwave user which transmits across thegeographic region of the base station. The PCN-base station can send acommand signal to the PCN unit through one of the spread-spectrumchannels, indicating which portions of spectrum to notch out with theadjustable-notch filter 725. A controller 726, which receives thecommand signal, can set the adjustable-notch filter 725 to one or morecenter frequencies and bandwidths. Note that this scenario assumes thatthe base station has knowledge of the location in frequency andbandwidths of the fixed-service, microwave channels operating within thesame geographic region of the cell.

Second, the PCN unit, or the PCN-base station, alternatively or inaddition, may have a sensor which detects the microwave power or energyof the one or more fixed-service, microwave channels. The sensordetermines the center frequency and the bandwidth of the fixed-servicemicrowave channel, and then the controller 726 adjusts theadjustable-notch filter 725 to notch the spread-spectrum-processed dataat those frequencies and bandwidths.

In a preferred embodiment the adjustable-notch filer 726 may be embodiedas an adaptive transversal filter. Any tunable notch filter, however,can be used for the adjustable-notch filter 726. A single-tuned,resistor-inductor-capactor, RLC, circuit has been found to suffice formany applications. The circuit may be tuned with a variable capacator,i.e., a varicap controlled by a voltage.

The spread spectrum signals of the present invention are designed to be"transparent" to other users, i.e., spread spectrum signals are designedto provide "negligible" interference to the communication of other,existing users. The presence of a spread spectrum signal is difficult todetermine. This characteristic is known as low probability ofinterception (LPI) and low probability of detection (LPD). The LPI andLPD features of spread spectrum allow transmission between users of aspread spectrum CDMA communications system without the existing users ofthe mobile cellular system experiencing significant interference. Thepresent invention makes use of LPI and LPD with respect to thepredetermined channels in the fixed-service microwave system. By havingthe power level of each spread spectrum signal below the predeterminedlevel, then the total power from all spread spectrum users within a celldoes not interfere with microwave users in the fixed-service microwavesystem.

The PCN units and optionally the base stations can have a notch filterin their respective transmitters for reducing the power transmitted fromthe PCN unit and base station at the frequency and fixed-servicebandwidth of the fixed-service microwave system. Accordingly, the PCNsystem, as disclosed herein, can overlay an already existingfixed-service microwave system without causing any interference to thefixed-service microwave system. The effect on the PCN system of notchinga portion of the bandwidth of the spread spectrum signal is minimalinasmuch as the notch removes only a small portion of the total power inthe spectrum of the spread spectrum signal. It has been foundexperimentally that the use of such filters does not noticeably affectthe acquisition time or the tracking capability of the system. Indeed,no deleterious affects were observed.

Spread spectrum is also "jam" or interference resistant. A spreadspectrum receiver spreads the spectrum of the interfering signal. Thisreduces the interference from the interfering signal so that it does notnoticeably degrade performance of the spread spectrum system. Thisfeature of interference reduction makes spread spectrum useful forcommercial communications, i.e., the spread spectrum waveforms can beoverlaid on top of existing narrowband signals. Accordingly, signalsfrom an already existing fixed-service microwave system cause negligibledegradation in performance of the spread-spectrum system.

The present invention employs direct sequence spread spectrum, whichuses a phase (amplitude) modulation technique. Direct sequence spreadspectrum takes the power that is to be transmitted and spreads it over avery wide bandwidth so that the power per unit bandwidth (watts/hertz)is minimized. When this is accomplished, the transmitted spread spectrumpower received by a microwave user, having a relatively narrowbandwidth, is only a small fraction of the actual transmitted power.

In a fixed-service microwave system, by way of example, if a spreadspectrum signal having a power of 1 milliwatt is spread over afixed-service microwave bandwidth of 48 MHz and a microwave user employsa communication system having a channel bandwidth of only 10 MHz, thenthe effective interfering power due to one spread spectrum signal, inthe narrow band communication system, is reduced by the factor of 48MHz/10 MHz which is approximately 5. Thus, the effective interferingpower is 1 milliwatt (mW) divided by 5 or 0.2 mW. For fifty concurrentusers of spread spectrum, the power of the interfering signal due tospread spectrum is increased by fifty to a peak interfering power of 10mW.

The feature of spread spectrum that results in interference reduction isthat the spread spectrum receiver actually spreads the received energyof any interferer over the same wide bandwidth, 50 MHz in the presentexample, while compressing the bandwidth of the desired received signalto its original bandwidth. For example, if the original bandwidth of thedesired PCN data signal is only 30 kHz, then the power of theinterfering signal produced by the cellular base station is reduced by50 MHz/30 kHz which is approximately 1600.

Direct sequence spread spectrum achieves a spreading of the spectrum bymodulating the original signal with a very wideband signal relative tothe data bandwidth. This wideband signal is chosen to have two possibleamplitudes, +1 and -1, and these amplitudes are switched, in a"pseudo-random" manner, periodically. Thus, at each equally spaced timeinterval, a decision is made as to whether the wideband modulatingsignal should be +1 or -1. If a coin were tossed to make such adecision, the resulting sequence would be truly random. However, in sucha case, the receiver would not know the sequence a-priori and could notproperly receive the transmission. Instead a chip-code generatorgenerates electronically an approximately random sequence, called apseudo-random sequence, which is known a-priori to the transmitter andreceiver.

To illustrate the characteristics of spread spectrum, consider 4800 bpsdata which are binary phase-shift keyed (BPSK) modulated. The resultingsignal bandwidth is approximately 9.6 kHz. This bandwidth is then spreadusing direct sequence spread spectrum to 16 MHz. Thus, the processinggain, N, is approximately 1600 or 32 dB.

Alternatively, consider a more typical implementation with 4800 bps datawhich is modulo-2 added to a spread-spectrum-chip-code signal, g_(i)(t), having a chip rate of 25 Mchips/sec. The resulting spread-spectrumdata are binary-phase-shift keyed (BPSK) modulated. The resultingspread-spectrum bandwidth is 50 MHz. Thus, the processing gain is:N'=(25×10⁶)/(4.8×10³), which approximately equals 5000, or 37 dB.

FIG. 5 shows the spectrum of this spread spectrum signal of an amplitudemodulated 3 kHz sinusoidal signal, when they each have the same powerlevel. The bandwidth of the AM waveform is 6 kHz. Both waveforms havethe same carrier frequency.

FIG. 6 shows the demodulated square-wave data stream. This waveform hasbeen processed by an "integrator" in the receiver, hence the triangularshaped waveform. Note that positive and negative peak voltagesrepresenting a 1-bit and 0-bit are clearly shown. FIG. 7 shows that thedemodulated AM signal replicates the 3 kHz sine wave.

The AM signal does not degrade the reception of data because the spreadspectrum receiver spreads the energy of the AM signal over 16 MHz, whilecompressing the spread spectrum signal back to its original 9.6 kHzbandwidth. The amount of the spread AM energy in the 9.6 kHz BPSKbandwidth is the original energy divided by N=1600 (or, equivalently, itis reduced by 32 dB). Since both waveforms initially were of equalpower, the signal-to-noise ratio is now 32 dB, which is sufficient toobtain a very low error rate.

The spread spectrum signal does not interfere with the AM waveformbecause the spread spectrum power in the bandwidth of the AM signal isthe original power in the spread spectrum signal divided by N₁, where##EQU1## hence the signal-to-interference ratio of the demodulated sinewave is 33 dB.

The direct sequence modes of spread spectrum uses psuedo randomsequences to generate the spreading sequence. While there are manydifferent possible sequences, the most commonly used are"maximal-length" linear shift register sequences, often referred to aspseudo noise (PN) sequences. FIG. 8 shows a typical shift registersequence generator. FIG. 9 indicates the position of each switch b_(i)to form a PN sequence of length L, where

    L=2.sup.N -1

The characteristics of these sequences are indeed "noise like". To seethis, if the spreading sequence is properly designed, it will have manyof the randomness properties of a fair coin toss experiment where"1"=heads and "-1"=tails. These properties include the following:

1) In a long sequence, about 1/2 the chips will be +1 and 1/2 will be-1..

2) The length of a run of r chips of the same sign will occur aboutL/2^(r) times in a sequence of L chips.

3) The autocorrelation of the sequence PN_(i) (t) and PN_(i) (t +τ) isvery small except in the vicinity of τ=0.

4) The cross-correlation of any two sequences PN_(i) (t) and PN_(i) (t+τ) is small.

Code Division Multiple Access

Code division multiple access (CDMA) is a direct sequence spreadspectrum system in which a number, at least two, of spread-spectrumsignals communicate simultaneously, each operating over the samefrequency band. In a CDMA system, each user is given a distinct chipcode. This chip code identifies the user. For example, if a first userhas a first chip code, g₁ (t), and a second user a second chip code, g₂(t), etc., then a receiver, desiring to listen to the first user,receives at its antenna all of the energy sent by all of the users.However, after despreading the first user's signal, the receiver outputsall the energy of the first user but only a small fraction of theenergies sent by the second, third, etc., users.

CDMA is interference limited. That is, the number of users that can usethe same spectrum and still have acceptable performance is determined bythe total interference power that all of the users, taken as a whole,generate in the receiver. Unless one takes great care in power control,those CDMA transmitters which are close to the receiver will cause theoverwhelming interference. This effect is known as the "near-far"problem. In a mobile environment the near-far problem could be thedominant effect. Controlling the power of each individual mobile user ispossible so that the received power from each mobile user is the same.This technique is called "adaptive power control". See U.S. Pat.Application having Filing Date of Nov. 16, 1990, entitled, "AdaptivePower Control Receiver," by Donald L. Schilling, which is incorporatedherein by reference.

The Proposed PCN Spread Spectrum CDMA System

The PCN spread spectrum communications system of this patent is a CDMAsystem. Direct Sequence Code Division Multiple Access (CDMA) cansignificantly increase the use of spectrum. With CDMA, each user in acell uses the same frequency band. However, each PCN CDMA signal has aseparate pseudo random code which enables a receiver to distinguish adesired signal from the remaining signals. PCN users in adjacent cellsuse the same frequency band and the same bandwidth, and therefore"interfere" with one another. A received signal may appear somewhatnoisier as the number of users' signals received by a PCN base stationincreases.

Each unwanted user's signal generates some interfering power whosemagnitude depends on the processing gain. PCN users in adjacent cellsincrease the expected interfering energy compared to PCN users within aparticular cell by about 50%, assuming that the PCN users are uniformlydistributed throughout the adjacent cells. Since the interferenceincrease factor is not severe, frequency reuse is not employed. Eachspread spectrum cell can use a full 48 MHz band for transmission and afull 48 MHz band for reception. Hence, using a chip rate of twenty fivemillion chips per second and a coding data rate of 32k bps results inapproximately a processing gain of 750 chips per bit. It is well knownto those skilled in the art that the number of PCN CDMA users isapproximately equal to the processing gain. Thus, up to 750 users canoperate in the 50 MHz bandwidth overlaying one or more fixed servicemicrowave systems in the 1.85-1.99 GHz region.

Shared Spectrum Capability of CDMA PCN

An interesting aspect of the use of DS CDMA for cellular radiotransmission is in the possibility of overlaying the DS CDMA PCN radionetwork on top of existing users occupying the frequency band ofinterest. That is, it is not necessary to supply to the spread spectrumusers a frequency band which is completely devoid of other users.Rather, if the frequency band is partially occupied by variousnarrowband users, it is often possible to superimpose the DS CDMAsignals on the same band in such a manner that both sets of users canco-exist.

A proposed PCN system geographic architecture is shown in FIG. 10. Amultiplicity of microcells each having a PCN-base station, communicatewith a plurality of PCN users.

To see that CDMA PCN can coexist with fixed service (FS) microwaveusers, the effect of the mobile PCN users on the FS microwave receiverand the effect of the FS microwave transmitter on a mobile PCN user mustbe examined.

Effect of PCN users on a FS Microwave Receiver

To examine the effect of the mobile PCN user on a FS microwave receiver,refer to FIG. 11. A PCN user is shown whose transmission is received bya microwave receiver. The PCN user's signal is attenuated by (1) pathloss and (2) antenna directivity which results in a significant decreasein the FS microwave antenna gain, FIG. 12, in the direction of the PCNuser.

For example, the link parameters for a typical 2 GHz FS link and for aPCN system are given in FIGS. 13A and 13B. The free space propagationloss, L_(uW), between FS transmitter and receiver is

    L.sub.uW =103+20 log(R), dB                                (1)

while the path loss LPCN between a PCN user and FS receiver typically isnot the free space path loss as it is affected by multipath. A standardrepresentation, approved by the CCIR is:

    L.sub.PCN =135.5+33.21 log(d), dB                          (2)

In these equations R is the distance, in miles, between transmitter andreceiver and d is the distance, in miles, between PCN and receiver, seeFIG. 11.

Using equations (1) and (2) and FIG. 13, the ratio of the receivedsignal power P_(s) from the FS microwave transmitter to the receivedinterference P_(I) of the PCN user(s) can be determined and is given theFIG. 14. In FIG. 14 it is assumed that multiple PCN users are allcongregated at the same location, clearly a worst-case result. It alsoshould be noted that if P_(s) /P_(I) =23 dB the probability of a symbolbeing in error before FEC decoding is 10⁻³. The coding gain of a typicalFS microwave receiver is 3 dB.

Assuming that there are 100 active PCN users/cell, uniformly distributedacross the cell, and there are 32 (or more) cells facing the FSmicrowave receiver, then the resulting P_(s) /P_(I) =53 dB, whichprovides a signal to noise ratio of 23 dB with a 30 dB fade margin. Thiscorresponds to an undecoded error rate of 10⁻³.

The addition of the notch filter at the PCN unit and/or PCN base stationsignifically reduces these already low values such that interferencewith a fixed-service microwave user is negligible or non existent. Whenthe spread-spectrum system overlaps the antenna bean and is near thereceiver of the fixed-service microwave system the notch filter providesmore than 15 dB additional attenuation to the spread-spectrum signalpower in the band of the notch. When the total power of thespread-spectrum system is spread over 48 MHz and the FS bandwidth isless than 10 MHz, only 20% of the spread-spectrum power is available tointerfere with the FS microwave system. Since most of the time thespread-spectrum PCN-base station and PCN unit are at a remote distancefrom a fixed-service microwave station, i.e. a fixed-service microwavestation is located outside the normal geographic coverage area of acell, the path loss from the PCN-base station or PCN user varies at anexponent greater than two, and typically by the fourth power. Also, mostof the time a PCN-base station and PCN user are not operating within theantenna beam of a fixed-service microwave station. Thus, the power ofthe spread-spectrum signal at the fixed-service microwave user isreduced by 20 dB to 40 dB.

Effect of a FS Microwave Transmitter on a PCN User

To calculate the effect of the FS microwave transmitter on PCN users,assume that there are 100 users uniformly distributed throughout eachcell and consider those cells "facing" the microwave transmitter. FIG.15 shows the region where the bit error rate, before FEC decoding,exceeds 10⁻². The dimensions of each cell are 1200 feet by 1200 feet.The area shown, therefore constitutes approximately 2.2% of the cellarea. Hence 2 to 3 users will be inconvenienced within that single cell.No users will be inconvenienced outside the region shown.

PCN Field Test

Experiments were conducted in the frequency band 1850-1990 MHz, toconduct field tests of a PCN system employing direct sequence spreadspectrum CDMA. The novel application seen here is that the band chosenfor experimentation is one which is used today for microwavetransmission. The field tests are intended to verify that spreadspectrum can share a band with existing users and thereby increase theutilization efficiency of a frequency band. These tests also provideimportant quantitative information, such as how many CDMA users and atwhat power level, can operate in the vicinity of a microwave receiverwithout degrading the microwave user's performance, and how many CDMAusers can operate in the vicinity of a microwave transmitter before theCDMA user's performance is degraded.

The field tests fall into two categories: measurement of theinterference produced by the spread spectrum PCN on the existingmicrowave users, and measurement of the interference produced by theexisting microwave users on both the mobile user and the cell. Theseexperiments were performed in New York and Orlando, Fla. during 1990 and1991.

FIG. 16 shows a typical, fixed location, existing microwavetransmitter-receiver site. The mobile users 1 and 2 each transmit to thecell using the frequency band 1860-1910 MHz and receive from the cellusing the band 1930-1980 MHz. Mobile user 3-50 is a transmit-only userwhich simulates 48 users transmitting from the same site. The powerlevel of mobile users 1 and 2 is adjustable from 100 uW to 100 mW, thepower level of mobile users 3-50 will be adjustable from 4.8 mW to 4.8W, and the power level of the cell is adjustable from 5 mW to 5 W. Eachadjustment is made independently of the others. Each mobile user had anotch filer located at the frequency and with a fixed-service microwavebandwidth of a fixed-service microwave user.

Measurement 1 Measurement of the Interference Product by PCN on ExistingMicrowave Receiver

The four vans, which include the mobile users as well as the cell, shownin FIG. 16, were moved relative to a microwave receiver, and the biterror rate (BER) measured at each position. The measured BERs arecompared to the interference-free BER obtained when the mobile system isoff. Different transmit powers from the cell and from the mobile usersare employed in order to determine the robustness of the system.

Measurements were taken during different times of the day and night, andat several receiver sites.

Measurement 2 Measurement of the Interface Produces by the ExistingMicrowave Transmitters on the PCN

The position of the four vans shown in FIG. 16 varied relative to theexisting microwave transmitters to determine the sensitivity of the PCto such interference. Both qualitative voice measurements andquantitative bit-error-rate measurements were made.

The robustness of the system to fading also was determined. Thismeasurement of the effect of the propagation characteristics of thechannel on the PCN were made by positioning the cell and mobile users indifferent parts of Orlando. BER measurements were taken, and acomparison to r², r³.6 and r⁴ curves were made in an attempt to bettercharacterize this PCN channel. FIGS. 17A-17K plot attenuation versusdistance based on these experimental results.

Fading Due to Multipath

The received waveform often includes numerous similar signals eachdelayed with respect to one another. This delay is due to the fact thatthe antenna transmits the same signal, with equal power, in alldirections simultaneously. Some of these signals, after bouncing off ofcars, buildings, roadways, trees, people, etc., are received after beingdelayed. These are called multipath signals. Thus, the total receivedsignal is: ##EQU2## where d_(i) is the data, g_(i) is the pseudo-noise(PN) sequence and ⊕ indicates modulo-2 addition.

If several τ_(i) are clustered together so that the difference betweenthe largest τ_(i) =τ_(k) and the smallest τ_(i) =τ₁, is less than theduration of a chip, i.e., τ_(k) -τ₁ <T_(c), then the received signalv_(R) (t) can be severely attenuated. This is called "fading" due tomultipath.

FIG. 18 shows the spectrum of a 24 Mchips/s direct sequence spreadspectrum signal at a carrier frequency of 1.956 GHz when multipathfading is present. Note that a 8 dB deep, 15 MHz wide fade can result.Other experiments performed indicate that typical fades are 10 dB orgreater and 1-3 MHz or more wide. Thus, a 48 MHz bandwidth, widebandspread spectrum signal is relatively insensitive to multipath fades,while "narrowband" signals having bandwidths of less than 3 MHz can begreatly attenuated due to fading.

Based on these findings personal communication networks according to thepresent invention using CDMA have numerous advantages as compared toFDMA and TDMA.

They can be used in a frequency band that has existing users, andtherefore this means of communication represents an effective, efficientmode of frequency band utilization.

Broadband-CDMA modulation is more robust in the presence of multipath.For example, if the direct path is 600 feet and the multipath is 800feet, the two returns are separated by 200 feet or 200 ns. Usingbroadband-CMDA modulation the chip rate of 25 Mchips/s means that thetwo returns are uncorrelated. Indeed, multipath returns exceeding 40feet are uncorrelated and do not result in fading.

CDMA has the potential of allowing a larger number of users, that is, ofbeing a more efficient system than either TDMA or FDMA. This improvementcan also be translated into lower power and hence longer life forbatteries.

In this decade, the CDMA PCN system is likely to be widely used forvoice communications, facsimile transmission and other types of datatransmission. Its versatility could well result in this system attaininga major share of the world's communication market.

It will be apparent to those skilled in the art that variousmodifications can be made to the spread spectrum CDMA communicationssystem using the notch filter of the instant invention without departingfrom the scope or spirit of the invention, and it is intended that thepresent invention cover modifications and variations of the spreadspectrum CDMA communications system using the notch filter provided theycome in the scope of the appended claims and their equivalents.

I claim:
 1. A spread spectrum CDMA communications system forcommunicating data, including digitized voice signals, between aplurality of personal-communications network (PCN) units, spreadspectrum communications system located within a same geographical regionas occupied by a microwave system using a microwave frequency andfixed-service bandwidth, said spread spectrum CDMA communications systemcomprising:a plurality of PCN-base stations located within the mobilecellular system for communicating data to the plurality of PCN units,each of said PCN-base stations including,base means for converting theformat of the data for output to communicate over radio waves;base-product means for spread spectrum processing the data; base-filtermeans for notching a spectrum of the spread-spectrum-processed-converteddata at the microwave frequency and microwave bandwidth of saidmicrowave system; and base means for transmitting across thefixed-service bandwidth, the spread-spectrum-processed-converted datafrom the PCN-base station to a PCN unit; a plurality of PCN units, eachof said PCN units including,a PCN antenna; and PCN detection meanscoupled to said PCN antenna and including PCN-spread-spectrum-processingmeans, for recovering data communicated from said PCN-base station. 2.The spread spectrum communications system as set forth in claim 1wherein said PCN detection means includes means for converting theformat of the data for output to a user.
 3. The spread spectrumcommunications system as set forth in claim 1 wherein saidPCN-spread-spectrum-processing means includes means for storing a localchip code for comparison to signals received for recovering data sentfrom said PCN-base station to said PCN unit.
 4. The spread spectrumcommunications system as set forth in claim 1 wherein saidPCN-spread-spectrum-processing means includes means for synchronizingsaid PCN-spread-spectrum-processing means to received signals.
 5. Thespread spectrum communications system as set forth in claim 1 whereinsaid PCN-spread-spectrum processing means at said PCN-base stationincludes means for processing data for particular PCN units with aselected chip code.
 6. The spread spectrum CDMA communications system asset forth in claim 1 wherein said base-filter means provides more than10 db attenuation in the fixed-service bandwidth.
 7. A spread spectrumCDMA communications system for communicating data, including digitizedvoice signals, between a plurality of personal-communications network(PCN) units, said spread spectrum communications system located within asame geographical region as occupied by a microwave system using amicrowave frequency and fixed-service bandwidth, said spread spectrumCDMA communications system comprising:a plurality of PCN units, locatedwithin a same geographical region as occupied by the microwave systemfor communicating data to a plurality of users, each of said PCN unitsincluding,PCN-product means for spread spectrum processing the data;PCN-filter means for notching a spectrum of thespread-spectrum-processed-converted data at the microwave frequency andthe microwave bandwidth of said microwave system; and PCN means fortransmitting across the fixed-service bandwidth, thespread-spectrum-processed-converted data from said PCN unit to aPCN-base station; a PCN-base station including,a base antenna; and basedetection means including base-spread-spectrum-processing means, forrecovering data communicated from said PCN unit.
 8. The spread spectrumcommunications system as set forth in claim 7 wherein said basedetection means includes means for converting the format of the data foroutput to a user.
 9. The spread spectrum communications system as setforth in claim 7 wherein said base-spread-spectrum-processing meansincludes means for storing a local chip code for comparison to signalsreceived for recovering data sent from said PCN-base station to said PCNunit.
 10. The spread spectrum communications system as set forth inclaim 7 wherein said base-spread-spectrum-processing means includesmeans for synchronizing said processing means to received signals. 11.The spread spectrum communications system as set forth in claim 7wherein said base-spread-spectrum-processing means at said PCN-basestation includes means for processing data for particular PCN units witha selected-chip code.
 12. The spread spectrum communications system asset forth in claim 7 wherein said means for transmitting transmits datain a time division format.
 13. The spread spectrum CDMA communicationssystem as set forth in claim 7 wherein said PCN-filter means providesmore than 10 dB attenuation in the fixed-service bandwidth.
 14. Aspread-spectrum transmitter for use with a PCN-system of a spreadspectrum CDMA communications system for communicating data, includingdigitized voice signals between a plurality of personal-communicationsnetwork (PCN) units, said spread spectrum communications system locatedwithin a same geographical region as occupied by a microwave system,said spread-spectrum transmitter comprising:means for converting theformat of the data for output to communicate over radio waves; productmeans for spread spectrum processing the data; filter means for notchinga spectrum of the data spread-spectrum-processed-converted at themicrowave frequency and microwave bandwidth of said microwave system;and means for transmitting across a bandwidth of said microwave system,the notched-spread-spectrum-processed-converted data.
 15. Thespread-spectrum transmitter as set forth in claim 14 wherein said notchmeans includes a notch filter having band rejection at the plurality ofpredetermined channels.
 16. The spread-spectrum transmitter as set forthin claim 14 wherein said notch means includes an adjustable-notch filterhaving band rejection at adjustable channels.
 17. The spread spectrumtransmitter as set forth in claim 15 wherein said filter means providesmore than 15 db attenuation in the bandwidth of said microwave system.